Impedance Control Network Resonant DC-DC Converters for Wide-Range High-Efficiency Operation

Lu, Jie

Graduate Thesis Or Dissertation

Impedance Control Network Resonant DC-DC Converters for Wide-Range High-Efficiency Operation Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/rx913q08r

Abstract

Grid-level power converters based on conventional architectures do not achieve very high efficiencies, and their efficiencies drop substantially as the operating conditions change. This thesis introduces and demonstrates a new resonant converter architecture that operates at fixed frequency and maintains zero-voltage switching (ZVS) and near zero-current switching (ZCS) across wide operating ranges in terms of input/output voltages and output power, minimizing device stresses and switching losses, and enabling both high efficiency and high power density. Unlike a conventional resonant converter, which utilizes a single inverter and a single rectifier, this Impedance Control Network (ICN) resonant converter has multiple inverters and one or more rectifiers. It also utilizes a lossless impedance control network, which provides a differential phase shift in the voltages and currents, whereby the effective impedances seen at the inverter outputs look purely resistive at the fundamental frequency, enabling switching of the inverters at zero current. By modifying the network for slightly inductive loading of the inverters, one can realize simultaneous ZVS and near-ZCS across wide operating ranges. This thesis also introduces a new modeling approach, termed step-superposition (S2) analysis, which enables exact modeling and optimization of high-order resonant converters. Three 200 W, 500 kHz step-up (25 V to 40 V input and 250 V to 400 V output) ICN resonant converter prototypes which are optimized using S2 analysis are designed, built and tested. One of these converters achieves a peak efficiency of 97.1%, and maintains greater than 96.4% full power efficiency at 250 V output voltage across its nearly 2:1 input voltage range. An optimized startup control approach is developed to further improve the efficiency of ICN converters operating under burst-mode control. This thesis also introduces an alternative to burst-mode control, termed enhanced phase-shift control, which reduces the output capacitance requirement by a factor of 100. A 120 W, 1 MHz step-down (18 V to 75 V input and 12 V output) ICN converter that demonstrates the advantages of this enhanced phase-shift control is designed, built and tested. Finally, a closed-loop control approach for output voltage regulation in ICN converters is introduced and its effectiveness is demonstrated.

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